Use of cerium oxide for removal of chlorine from fuel gases

ABSTRACT

A process for the removal of chlorine from reducing gases is provided wherein the reducing gases are contacted with cerium oxide. The chlorided cerium oxide is regenerated in an oxidizing atmosphere to provide cerium oxide. This cerium oxide regenerated is again capable of reacting with chlorine in reducing gases.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the use of cerium oxide (CeO₂) for the removal of chlorine (Cl₂) from reducing gases. The cerium chloride (CeCl₃) generated by the reaction of the Cl₂ with CeO₂ can be regenerated back to CeO₂ by exposure to an oxidizing atmosphere such as air with the release of Cl₂.

2. Description of the Prior Art

For purposes of illustration the removal of chlorine from fuel gases generated in a gasifier from hydrocarbons which also contain either Cl₂ or compounds which contain chlorine that dissociate with the release of Cl₂ will be used. Many of the hydrocarbons that may be used for the creation of fuel gases also contain sulfur which is found in the fuel gas mainly in the form of hydrogen sulfide (H₂ S) or carbon oxysulfide (COS). Patents currently exist for the removal of sulfur from fuel gases with cerium oxide and doped cerium oxide, and thermodynamic calculations indicate that the removal of H₂ S, COS, and Cl₂ can occur simultaneously. However, the use of fuel gases produced from hydrocarbons containing both sulfur and chlorine for illustrative purposes does not preclude the use of the technology for the reduction of Cl₂ from other reducing gases containing Cl₂ no matter what their method of preparation or production.

One of the major applications of the technology may be the removal of Cl₂ from the fuel gases used in integrated-gasifier-combined cycle (IGCC) power plants which may become the preferred method of electric power generation in the future. The gasifiers of IGCC systems may operate at temperatures as high as 2800° F. Undesirable components of the fuel gases produced in the gasifier may be removed by lowering the temperature of the fuel gases to a temperature either slightly above or slightly below the boiling point of water by methods known to those skilled in the art and then exposing the fuel gases to chemicals and water solutions of chemicals capable of removing the undesirable components such as Cl₂ and H₂ S from the gases. Because of the almost complete loss of the sensible heat from the gases, the cost of electricity (COE) of power plants which utilize this technology for removal of undesirable components from the fuel gases produced by gasifiers has been computed to be about 10% higher than the cost of IGCC systems that could remove the undesirable components such as H₂ S and Cl₂ at high temperatures. Although methods have been developed which operate at close to the operating temperature of the gasifier for removing H₂ S from fuel gases by exposing them to materials such as zinc ferrite, zinc titanate, copper-manganese oxide mixtures, and cerium oxide, an extensive literature review reveals no studies of methods for the use 9f solid sorbents to remove Cl₂ from fuel gases.

Data available on the analysis of coals produced in Illinois show that the chlorine content of those coals can range from 0.02% to 0.42% with the average being 0.112%. The sulfur content of these same coals ranges from 1.14% to 4.52% with the average being 3.31%. When such coals are used in an oxygen blown gasifier the H₂ S content of the resulting gases based on their average sulfur content would be about 1.00%. If all of the chlorine in the coal is converted to Cl₂, the Cl₂ content of the fuel gases produced from the coal whose chlorine content is 0.42% could be as high as 0.10% or 1000 ppm.

In an IGCC system the fuel gases from the gasifier are fired into a gas turbine, and it is recognized that the blades in a gas turbine may operate at temperatures as high as 2000° F. (1093.3° C.). When the temperature of the blades in a gas turbine approaches 2000° F. in the oxidizing atmosphere created by burning the fuel gases, the blades may be partially oxidized. As a result of this oxidation there is a film of oxide coating the blades. The oxides on turbine blades are in many cases either high in Al₂ O₃ or Cr₂ O₃ because many turbine blade alloys contain aluminum and chromium.

                  TABLE I                                                          ______________________________________                                         SUMMARY OF MELTING POINT AND VAPOR                                             PRESSURE                                                                       Fluorides       Chlorides    Oxides                                            mp         tv       mp      tv            mp                                   ______________________________________                                         FeX.sub.2                                                                              1020   906      676   536    FeO    1378                               FeX.sub.3                                                                              1027   673      303   167    Fe.sub.2 O.sub.3                                                                      1594                               NiX.sub.2                                                                              1450   939      1030  607    NiO    1955                               CoX.sub.2                                                                              1250   962      740   587    CoO    1805                               CrX.sub.2                                                                               894   928      820   741                                              CrX.sub.3                                                                              1404   855      1150  611    Cr.sub.2 O.sub.3                                                                      2400                               AlX.sub.3                                                                              1273   825      193    67    Al.sub.2 O.sub.3                                                                      2050                               ______________________________________                                          mp = melting point (°C.)                                                tv = temperature at which v.p = 10.sup.-4 atmospheres                    

The data in Table I shows that all of the chlorides of the major constituents of turbine blades have substantial vapor pressures at temperatures less than the operating temperatures expected in many gas turbines with AlCl₃ having a very low melting point and a very high vapor pressure. As a result, there will be a continuous vaporization of the chlorides formed from the oxides created on the turbine blades when the fuel gases contain Cl₂. This will result in premature failure of the turbine blades.

A need therefore exists for a method for removing chlorine from fuel gases produced from coals containing chlorine containing compounds or chlorine itself.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method for removing chlorine from fuel gases created from hydrocarbons containing chlorine or chlorine bearing compounds in a gasifier which contain little or no sulfur either as H₂ S or COS. The chlorine content of resulting gases is reduced to a level sufficiently low that the expected life of gas turbine blades may be achieved.

Another object of the present invention is to provide a method for the simultaneous removal of hydrogen sulfide (H₂ S), carbon oxysulfide (COS) and chlorine (Cl₂) from fuel gases created from hydrocarbons containing sulfur and sulfur containing compounds and chlorine and chlorine containing compounds to a level sufficiently low that the expected life of the gas turbine blades may be achieved.

Briefly, these objects can be achieved by exposing fuel gases containing Cl₂ alone or fuel gases containing sulfur compounds such as H₂ S and COS and Cl₂ to cerium oxide whereby the Cl₂ content of the gases will be reduced to a level sufficiently low that the expected life of the gas turbine blades may be achieved. The sulfur content of the gases is also reduced to a level sufficiently low to meet present and future requirements of the Environmental Protection Agency (EPA) for sulfur emissions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention may be explained using thermodynamic calculations which predict the extent of capture of Cl₂ with CeO₂ both when there is no sulfur present and when there is sulfur present in the gases.

The first embodiment is the removal of Cl₂ from reducing gases when there is no sulfur present.

Two of the equations related to the removal of chlorine from fuel gases are:

    Ce(s)+O.sub.2 (g)=CeO.sub.2 (s)                            (1)

and

    Ce(s)+3/2Cl.sub.2 (g)=CeCl.sub.3 (g)                       (2)

When the two equations are combined and the terms rearranged the equation for the reaction of CeO₂ and 2Cl₂ can be written:

    CeO.sub.2 (s)+3/2Cl.sub.2 =CeCl.sub.3 (s)+O.sub.2 (g)      (3)

The equilibrium constant "K" for this equation can be written:

    K=((a)CeCl.sub.3 ×pO.sub.2)/((a)CeO.sub.2 ×[pCl.sub.2 ].sup.3/2)(4)

Since CeCl₃ and CeO₂ are solids their activity (a) is one. The equation can be rewritten as:

    K=pO.sub.2 /(pCl.sub.2).sup.3/2                            (5)

From the thermodynamic data available for the equations (1), (2) and (3) the equilibrium constant "K" can be calculated to be 9.74×10⁻⁴ at a temperature of 1273° K. (1000° C.) [1832° F.], and equation (5) can be rewritten as:

    9.74×10.sup.-4 =pO.sub.2 /(pCL.sub.2).sup.3/2        (6)

When the pO₂ (3.697×10⁻¹⁷) in equilibrium with a gas from a Prenflo gasifier (assuming no sulfur in the gas) is substituted in equation (6) the pCl₂ can be calculated to be 7.129×10⁻⁹, virtually complete elimination of all Cl₂ from the fuel gas.

In the same manner when the pO₂ (1.579×10⁻¹⁴) in equilibrium with a fuel gas from a KRW gasifier (assuming no sulfur in the gas) is substituted in equation (6) the pCl₂ can be calculated to be 6.454×10⁻⁸, again virtually complete elimination of the chlorine.

From these computations it can be seen that the thermodynamic calculations predict virtual elimination of chlorine from fuel gases produced from the most efficient and the least efficient gasifiers.

The effect of temperature and gas composition on the removal of chlorine from reducing gases is illustrated in Table II.

                  TABLE II                                                         ______________________________________                                         Temperature  Prenflo Gas KRW Gas                                               ______________________________________                                         1881° F.                                                                             1.025 × 10.sup.-9                                                                    5.866 × 10.sup.-8.sup.                          1521° F.                                                                             .sup. 8.212 × 10.sup.-12                                                             5.676 × 10.sup.-10                               981° F.                                                                             2.858 × 10.sup.-8                                                                    1.370 × 10.sup.-15                              ______________________________________                                    

It is also to be noted that the melting point of CeCl₃ is 848° C. (1558° F.) and its boiling point is 1727° C. (3141° F.).

The ability to regenerate any CeCl₃ formed may be assessed by the partial pressure of chlorine when the regenerating medium is air whose pO₂ is 0.21. Computations based on equation (6) at 1273° K. (1832° F.) show that under those conditions the partial pressure of Cl₂ is 111.9 atmospheres which should assure complete regeneration of any CeCl₃ formed by exposure of CeO₂ to chlorine containing fuel gases.

Likewise, thermodynamic calculations can be utilized to predict the extent of Cl₂ removal from fuel gases which contain sulfur.

During desulfurization and dechlorination of a fuel gas, two competing reactions may take place:

    2CeO.sub.2 (s)+1/2S.sub.2 (g)=Ce.sub.2 O.sub.2 S(S)+O.sub.2 (g)(7)

and

    CeO.sub.2 (s)+3/2Cl.sub.2 (g)=CeCl.sub.3 (s)+O.sub.2 (g)   (8)

At 1300° K. (1700° F.) the equilibrium constant "K" for equations (7) and (8) can be calculated. It is also to be noted that both equations (7) and (8) contain a term for pO₂. Since the reactions take place together, the pO₂ content in both equations (7) and (8) is the same. Therefore, three additional equations can be written:

    pO.sub.2 (g)=K.sub.7 (pS.sub.2).sup.1/3                    (9)

and

    pO.sub.2 (g)=K.sub.8 (pCl.sub.2).sup.3/2                   (10)

and

    K.sub.7 (pS.sub.2)1/2=K.sub.8 (pCl.sub.2).sup.3/2          (11)

When various values of pS₂ are substituted in equation (11) the corresponding pCl₂ can be calculated assuming a desulfurization temperature of 1300° K. (1880° F.) and 1000° K. (1341° F.). These values shown in Table III show the reduction of pCl₂ as the temperature at which the Cl₂ is removed from the gas is reduced.

                  TABLE III                                                        ______________________________________                                         ppm H.sub.2 S                                                                              pCl.sub.2 (1880° F.)                                                                pCl.sub.2 (1341° F.)                            ______________________________________                                         15000       1.08 × 10.sup.-6                                                                     1.186 × 10.sup.-9                                 300        3.01 × 10.sup.-7                                                                     3.280 × 10.sup.-10                                 5         7.68 × 10.sup.-8                                                                     8.380 × 10.sup.-11                               ______________________________________                                    

If an Illinois #6 coal is gasified in an oxygen blown gasifier with no in-bed desulfurization the H₂ S content is close to 15000 ppm (1.5% H₂ S) and the chlorine content may be as high as 300 ppm (0.030% Cl₂). If that gas is exposed to CeO₂, the results shown in Table III above indicate that the Cl₂ content is expected to be approximately 1 ppm when in equilibrium with 15,000 ppm H₂ S, and as the pH₂ S decreased the pCl₂ would also decrease. It is expected that these levels of Cl₂ would meet the requirements of gas turbine manufactures.

If the temperature of desulfurization was lowered to 1000° K. (1341° F.) the pCl₂ in equilibrium with 15,000 ppm H₂ S would be 7.186×10⁻⁹.

In summary, thermodynamic calculations predict the reduction in Cl₂ from fuel gas streams created from Illinois #6 coal even in the presence of 15,000 ppm H₂ S at 1880° F. to 1 ppm when CeO₂ is used for desulfurization of fuel gases, and to lower levels as the temperature is reduced or as the amount of H₂ S present is reduced. The CeCl₃ created by this reduction of Cl₂ from the fuel gas can be eliminated by exposing the Ce₂ O₂ S-CeCl₃ mixture to air at the same temperatures used for regeneration of the Ce₂ O₂ S.

The ability to regenerate any CeCl₃ formed when regenerating Ce₂ O₂ S formed by desulfurization of the gases may be assessed by the partial pressure of chlorine when the regenerating media is air whose pO₂ is 0.21. Computations based on equation (6) show that under those conditions the partial pressure of Cl₂ is 111.9 atmospheres, which should assure complete regeneration of any CeCl₃ formed by exposure of CeO₂ to chlorine containing fuel gases. If the temperature of regeneration is above 925° C. there should also be complete regeneration of any Ce₂ O₂ S.

Various embodiments and modifications of this invention have been described in the foregoing description, and further modifications will be apparent to those skilled in the art. Such modifications are included within the scope of the invention as defined by the following claims. 

We claim:
 1. A process for the removal of chlorine from reducing gases comprising the steps of:(a) contacting said reducing gases containing chlorine with cerium oxide to form chlorided cerium oxide; (b) removing said chlorided cerium oxide from said reducing gases; (c) regenerating cerium oxide by exposing the chlorided cerium oxide to an oxidizing atmosphere to provide cerium oxide which is again capable of reacting with chlorine; and (d) returning said regenerated cerium oxide into contact with said reducing gases.
 2. The process of claim 1 wherein the temperature is lowered to achieve more complete removal of the chlorine from the reducing gas.
 3. The process of claim 1 wherein the reducing power of the gases is increased to achieve more complete removal of chlorine from the reducing gas.
 4. The process of claim 1 wherein said reducing gases contain sulfur and said cerium oxide forms chlorided cerium oxide and sulfated cerium oxide upon contact with said reducing gases and said sulfated cerium oxide is removed from said reducing gases and said cerium oxide is regenerated by exposing the sulfated cerium oxide to air.
 5. The process of claim 4 wherein the reducing power of the reducing gases is increased to achieve more complete removal of the chlorine from the reducing gas.
 6. The process of claim 4 wherein the temperature is lowered to achieve more complete removal of chlorine from the reducing gas.
 7. The process of claim 4 wherein the amount of sulfur in the reducing gas is lowered to achieve more complete removal of the chlorine from the reducing gas. 